1. Field of the Invention
The present invention relates to circuit construction of a tuning circuit to improve frequency selectivity thereof by using a negative resistance circuit.
2. Description of the Related Art
As a method for improving frequency selectivity of a tuning circuit there are methods for improving Q of the tuning circuit by a negative resistance circuit and one of them is a method for compensating equivalent series resistance component of a tuning circuit by constituting it so that an inductor is connected to a capacitor in series and an input signal are applied to one side thereof, and the other side is earthed through a negative resistance circuit. Compensation of the equivalent series resistance component is done to obtain the following two merits. That is, one of them is a merit able to keep a 3 dB band width constant because a value of ωL is changed by changing a tuning frequency but Q changes in proportion to a frequency (Q=ωL/r) since an equivalent series resistance component r does not change and the other is a merit that if a current component is taken out as an output of a tuning circuit, a maximum value at a tuning point, that is, a circuit gain of the tuning circuit is constant in dependently to frequency.
A negative resistance is provided by an amplifier having a positive feedback circuit and thus a negative resistance circuit can be constituted by the positive feedback circuit, but the circuit becomes unstable due to existence of the positive feedback circuit. In order to suppress this and obtain a stable positive feedback operation, a negative feedback circuit as well as a positive feedback circuit should be used. Circuits according to various methods such as a negative resistance circuit are known, but circuits of which design and adjustment are easy, suitable operation in a high frequency band is possible, and circuit construction is simple are very few. So, the inventor has filed Japanese Patent Application No. 2002-218036 as a construction plan of a tuning circuit, which corresponds to U.S. Patent Application Ser. No. 10/454,724, filed on Jun. 4, 2003 and published as U.S. Patent Publication No. 2004/0017273.
The tuning circuit of the above application includes a differential amplifying circuit having two transistors of which emitters are directly coupled and a low impedance output circuit such as an emitter follower. An output signal of the emitter follower is fed back to a same phase input side of the differential amplifying circuit directly and further is fed back to an inverse phase input side through a negative resistance value setting resistor therefrom to obtain a negative resistance between a inverse phase input terminal and an earth. Since this circuit is constituted by only well known usable circuits, circuit design and adjustment thereof is easy and it operates stably up to a comparatively high frequency because of a simple circuit.
However, since this negative resistance circuit requires at least three transistors, in order to make it operable up to further high frequency, it is necessary to make its circuit construction simple so as to reduce phase rotation quantity in the inside thereof and for this reason it is desirable to reduce the number of transistors further.